Cold Spray: Over 30 Years of Development Toward a Hot Future

Abstract

Cold Spray (CS) is a deposition process, part of the thermal spray family. In this method, powder particles are accelerated at supersonic speed within a nozzle; impacts against a substrate material triggers a complex process, ultimately leading to consolidation and bonding. CS, in its modern form, has been around for approximately 30 years and has undergone through exciting and unprecedented developmental steps. In this article, we have summarized the key inventions and sub-inventions which pioneered the innovation aspect to the process that is known today, and the key breakthroughs related to the processing of materials CS is currently mastering. CS has not followed a liner path since its invention, but an evolution more similar to a hype cycle: high initial growth of expectations, followed by a decrease in interest and a renewed thrust pushed by a number of demonstrated industrial applications. The process interest is expected to continue (gently) to grow, alongside with further development of equipment and feedstock materials specific for CS processing. A number of current applications have been identified the areas that the process is likely to be the most disruptive in the medium-long term future have been laid down.

Keywords: Cold spray; Hype cycle; Innovation; Powder deposition.

Fig. 1

Working mechanism of a typical cold spray system

Fig. 2

Apparatus for impacting one metal upon another modified by (Ref 10)

Fig. 3

Indicative drawings of Schoop's invention, modified by (Ref 14)

Fig. 4

Device for treating the surface of a workpiece as modified by (Ref 15). Sectional views of the a) device, b) nozzle, c) liquid container, d) dust separator, e) arrangement of fluid containers, f) filter bags and housing inlet

Fig. 5

The first apparatus (Ref 21) for the fabrication of cold spray coatings (granted in 1991, Soviet Union)

Fig. 6

Various nozzle designs as proposed by (a) Alkhimov (Ref 29), (b) Dikun (Ref 30), (c) Krysa (Ref 31), and (d) Kashirin (Ref 32)

Fig. 7

Early-stage patents for cold spray with gas heating: (a) Gas powder supplied from the same line, (b) Powder and gas supplied from separate lines

Fig. 8

(a) Critical velocity of various materials; (b) Correlation between particle velocity, deposition efficiency and impact effects for a constant impact temperature, successful bonding occurs at deposition window; (c) Critical velocity and impact velocity over particle impact temperature; (d) Critical velocity and impact velocity over particle size (Ref 16, 75)

Fig. 9

Impact energy of different material deposited onto polymer substrates (Ref 78)

Fig. 10

(a) Pulsed-gas dynamic spraying system and its working principal; (b) Microstructure of typical P-GDS coatings (Cu, Zn, Al, Al-12Si, and nanocrystalline WC-15Co) deposited onto Al substrate (Ref 54)

Fig. 11

Laser-assisted cold spray system (Ref 85)

Fig. 12

(a) Schematic view of supersonic and sonic micro-nozzles; (b) Cu deposits on Al substrate sprayed by supersonic and sonic micro-nozzles (Ref 101, 102)

Fig. 13

The illustration of in-situ shot peening-assisted cold sprayed deposition mechanism (Ref 104)

Fig. 14

Cross-sectional microstructures of the titanium coatings deposited with pure titanium powder and powder mixtures with different proportions of shot peening particles (Ref 105)

Fig. 15

(a) Bright field-transmission electron microscope image with EDX mapping and (b) XRD patterns at the interface between AZ31B Mg alloy substrate and AA7075 alloy deposit (Ref 109)

Fig. 16

(a) Experimental setup of the microparticle impact test and real-time high-speed imaging system; (b)-(e) Multi-frame sequences with 5 ns exposure times showing the process of tin particles approaching and impacting onto tin substrate at increased velocity, spanning from the rebound regime to the bonding and the erosion regimes; (f) Coefficient of restitution, v_r/v_i, of the rebounding tin particles and fragments. (Ref 110)

Fig. 17

(a) Deformation of particle upon impact and break-up of oxide films (Ref 112, 113); (b) SEM images and AES mapping of fracture interface between substrate craters and deposition particles (Ref 112)

Fig. 18

Cross-sectional microstructure, tensile properties, and fatigue crack growth rate of the cold sprayed 316L deposits before and after heat treatments (Ref 114)

Fig. 19

Stages and key indicators of hype cycle curve (Ref 115)

Fig. 20

Indicative hype cycle of the cold spray technique

Fig. 21

The water repellency of cold sprayed Cu-based superhydrophobic coating: (a) the superhydrophobic performance on different substrate materials including (a1) Al alloy, (a2) Mg Alloy, (a3) Al2O3 ceramic and (a4) optical glass; (b) the results of water contact angle and water sliding angle on different substrates; (c) self-cleaning test result by using sticky starch powders; (d) the enhanced floating ability of different plates with superhydrophobic coating (Ref 244)

Fig. 22

Cold sprayed FeCoNiCrMn HEA coating: (a) image of the HEA coating on the substrate, (b) XRD spectra of the HEA powder and coating, (c) cross-sectional SEM image of the HEA coating, EBSD IPF maps of (d) a single HEA particle, and (e) the cold sprayed HEA coating (Ref 370)

Fig. 23

Schematic illustration of two typical cold sprayed MMC coating formation mechanisms using: (a) blended feedstock and (b) satellited feedstock (Ref 384)