Tantalum and Niobium Powders for Cold Spray Applications
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 paper below provides details of the advancement of tantalum and niobium cold spray powders produced by GAM and in support of the emerging cold spray applications.
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 Spray1 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 schematic2 .
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 post-repair heat treatment. Eliminating thermal treatments permits point-of-use repair, making cold spray extremely attractive in applications such as keeping military vehicles in service3 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 obsolescence5. Cold spray is demonstrating the unique ability to combine materials with different properties (e.g., yield strength, melting point, corrosion resistance) to form graded layers6 or a blended mixture such as a hard-phase / soft-phase composite for the replacement of electroplated hard coatings7. Cold spray processing and materials have been developed for aerospace8, a large number of military applications9 10, and to address extreme environments in energy generation11 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 applications14 and its use continues growing due to its low toxicity15 16, osteointegration capability17, and radiopacity18. In military applications, tantalum is used in durable electronics capacitors, munitions19, and has also been tested to extend the life of gun barrels20. Tantalum fills important roles in defense and is considered a critical strategic material21.
A review of literature shows continued testing of cold sprayed Ta and Nb in critical military22 and energy applications23. 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 properties22 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 usability26. For a given material, powder flowability is a function of particle size distribution (PSD), shape, and surface. Narrower size distributions flow more readily27; equiaxed shapes like spheres and cubes flow easier than irregular particles28 29; and rough, dirty, or damp surfaces increase interparticle friction and inhibit motion26 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 velocity31 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.
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 material32.
Higher gas impurities increase the metal’s yield strength and thereby increase the critical velocity33 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.
The surface of a particle influences its flowability (see above), the deposition efficiency, and coating properties26 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-to-volume 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.
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: 50×50 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
1- V. Champagne Jr, O. Ozdemir and A. Nardi, Practical Cold Spray, Springer, 2021.
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].
3- T. Pelsoci, “ESTCP funded cold spray technology for DOD applications,” 2015.
4- C. Munro, “Review of structural additive manufacturing for defence applications,” Defence Research and Development Canada, 2018.
5- J. Conant, “Army researchers develop cold-spray system, transition to industry,” Adelphi, 2015.
6- A. DeBiccari and J. Haynes.US Patent 8349396B2, 2013.
7- V. Champagne, A. Nardi and I. Nault, “Cold spray coatings for Cr and Ni plating replacement,” 2019.
8- J. Haynes and J. Karthikeyan, “Cold spray copper application for upper stage rocket engine design,” in ITSC, Orlando, 2003.
9- Materials Deposition, Cold Spray MIL-STD 3021, United States Dept. of Defense, 2008.
10- P. Leyman and V. Champagne, “Cold spray process development for the reclamation of the Apache helicopter mast support,” Army Research Laboratory, 2009.
11- A. Parsi, J. Lareau, B. Gabriel and V. Champagne, “Cold spray coatings for prevention and mitigation of stress corrosion cracking,” in CSAT, Worcester, 2012.
12- M. Showalter, “Cold Spray to Help Keep Turbines Spinning,” 6 10 2021. [Online]. Available: https://www.pnnl.gov/news-media/cold-spray-help-keep-turbines-spinning.
13- P. Lu, “Corrosion resistant cold sprayed cladding for molten salt reactors,” in PRiME, 2020.
14- E. Fusco, “The repair of hiatus hernia with tantalum mesh,” Military Medicine, vol. 125, no. 3, pp. 189-190, 1960.
15- H. Tang, K. Yang, L. Jia, W. He, L. Yang and X. Zhang, “Tantalum bone implants printed by selective electron beam manufacturing (SEBM) and their clinical applications,” JOM, pp. 1016-1021, 2020.
16- J. Tang, Z. Zhao, X. Cui, J. Wang and T. Xiong, “Microstructure and bioactivity of a cold sprayed rough/porous Ta coating on Ti6Al4V substrate,” Sci. China: Technol. Sci., pp. 731-739, 2020.
17- S. Kumar and B. Stucker, “Development of a Co-Cr-Mo to Ta transition using LENS for orthopedic applications,” in International Solid Freeform Fabrication Symposium, Austin, 2005.
18- ASM Materials for Medical Devices Database Committee, “Tantalum,” in Materials and Coatings for Medical Devices: Cardiovascular, ASM International, 2009, pp. 124-129.
19- D. Hambling, “The U.S. Army wants some tank-busting silver bullets,” Popular Mechanics, 20 1 2016.
20- S. Lee, J. Cox, D. Windover, M. Audino and D. Matson, “High-rate sputter-deposited Ta coatings on a steel liner for wear and erosion mitigation,” Surf. Coat. Technol., vol. 149, 2001.
21- Committee on Armed Services House of Representatives, “National Defense Authorization Act for Fiscal Year 2020,” 2020.
22- M. Trexler, V. Champagne and J. Kopchik, “Development of a novel erosion resistant coating system for use on rotorcraft blades,” in AHS International 68th Annual Forum & Technology Display, Fort Worth, 2012.
23- PaintSquare News, “DOE Awards $1.1M for Cold Spray Research,” 2021.
24- H. Koivuluoto, P. Vuoristo and J. Nakki, “Structure and corrosion behavior of cold-sprayed tantalum coatings,” in ITSC, Las Vegas, 2009.
25- S. Paul and P. McNutt, “Corrosion performance of cold sprayed tantalum coatings,” in CORROSION 2018, Phoenix, 2018.
26- B. Young, J. Heelan, S. Langan, M. Siopis, C. Walde and A. Birt, “Novel characterization techniques for additive manufacturing powder feedstock,” Metals, 2021.
27- D. Helfritch and V. Champagne, “Optimal particle size for the cold spray process,” in ITSC, Seattle, 2006.
28- O. Ozdemir, J. Conahan and S. Muftu, “Particle velocimetry, CFD, and the role of particle sphericity in cold spray,” Coatings, 2020.
29- M. Rokni, S. Nutt, C. Widener, V. Champagne and R. Hrabe, “Review of relationship between particle deformation-coating microstructure and properties in high pressure cold spray,” J. Therm. Spray Technol., pp. 1-40, 2017.
30- B. Sousa, M. Gleason, B. Haddad, V. Champagne Jr., A. Nardi and D. Cote, “Nanomechanical characterization for cold spray: from feedstock to consolidated material properties,” Metals, vol. 10, 2020.
31- R. Huang and H. Fukanuma, “Study of the influence of particle velocity on adhesive strength of cold spray deposits,” J. Therm. Spray Technol., pp. 541-549, 2011.
32- Y. Li, X. Luo and C. Li, “Dependency of deposition behavior, microstructure and properties of cold sprayed Cu on morphology and porosity of the powder,” Surface and Coatings Technology, 2017.
33- L. Descurninges, L. Mingault, J. M., A. Nifa, L. Berthe, M. Boustie, T. Laguionie, L. Bianchi and K. Ogawa, “Influence of powder particle oxidation on the properties of cold-sprayed tantalum,” in ITSC, Hamburg, 2011.
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].
For more information contact:
Gordon Smith
Chief Technology Officer
e: GCSmith@globaladvancedmetals.com
1223 County Line Road, Boyertown, PA, 19512, US
Ryan Clement
Chief Commercial Officer
e: RClement@globaladvancedmetals.com
100 Worcester Street, Suite 200, Wellesley, MA, 02481, US
DISCLAIMER: The information contained in this document was obtained from third party sources. GAM has not independently tested or confirmed the accuracy or conclusions of these sources and GAM is not responsible for any errors or omissions, or for the results obtained from the use of the information provided in this article and the articles cited below. All information in this article is provided “as is”, with no guarantee of completeness or accuracy of the results obtained from the use of the information in this article and the articles cited below, and without any kind of warranty, express or implied.