Introduction

V.K. Champagne    US Army Research Laboratory, USA

Cold spray is a process whereby metal powder particles are utilized to form a coating by means of ballistic impingement upon a suitable substrate.1−3 The metal powders range in particle size from 5 to 100 μm and are accelerated by injection into a high-velocity stream of gas. The high-velocity gas stream is generated through the expansion of a pressurized, preheated gas through a converging–diverging nozzle. The pressurized gas is expanded to supersonic velocity, with an accompanying decrease in pressure and tem- perature.4−6 The powder particles, initially carried by a separate gas stream, are injected into the nozzle either prior to the throat or downstream of the throat. The particles are then accelerated by the main nozzle gas flow and are impacted onto a substrate after exiting the nozzle. Upon impact, the solid particles deform and create a bond with the substrate.7,8 As the process continues, particles continue to impact the substrate and form bonds with the deposited material, resulting in a uniform coating with very little porosity and high bond strength. The term ‘cold spray’ has been used to describe this process due to the relatively low temperatures (− 100 to + 100 °C) of the expanded gas stream that exits the nozzle.

Cold spray as a coating technology was initially developed in the mid 1980s at the Institute for Theoretical and Applied Mechanics of the Siberian Division of the Russian Academy of Science in Novosibirsk.9 The Russian scientists successfully deposited a wide range of pure metals, metallic alloys, polymers, and composites onto a variety of substrate materials, and they demonstrated that very high coating deposition rates are attainable using the cold spray process. These experiments are described in detail in Chapter 2. Currently, a variety of cold spray research is being conducted at institutions in dozens of locations world-wide.

The temperature of the gas stream is always below the melting point of the particulate material during cold spray, and the resultant coating and/or freestanding structure is formed in the solid state. Since adhesion of the metal powder to the substrate, as well as the cohesion of the deposited material, is accomplished in the solid state, the characteristics of the cold spray deposit are quite unique in many regards. Because particle oxidation is avoided, cold spray produces coatings that are more durable with better bond strength. The exceptional adhesion of cold spray coatings is in part due to the low temperatures at which the coatings are deposited. One of the most deleterious effects of depositing coatings at high temperatures is the residual stress that develops, especially at the substrate-coating interface. These stresses often cause debonding. This problem is compounded when the substrate material is different from the coating material. This problem is minimized or eliminated when cold spray is used. In addition, interfacial instability due to differing viscosities and the resulting roll-ups and vortices promote interfacial bonding by increasing the interfacial area, giving rise to material mixing at the interface and providing mechanical interlocking between the two materials.

A key concept in cold spray operation is that of critical velocity.10,11 The critical velocity for a given powder is the velocity that an individual particle must attain in order to deposit (or adhere) after impact with the substrate. Small particles achieve higher velocities than do larger particles, and, since powders contain a mixture of particles of various diameters, some fraction of the powder is deposited while the remainder bounces off. The weight fraction of powder that is deposited divided by the total powder used is called the deposition efficiency. High velocity is necessary for optimal deposition efficiency and packing density, and several parameters – including gas conditions, particle characteristics, and nozzle geometry – affect particle velocity. Chapters 7 through 10 consider the effects of these parameters in detail.

The two principal cold spray system configurations differ in the location of powder injection into the nozzle, and these systems are described in more detail in Chapters 11 and 12. The two configurations are depicted by Figs 1.1 and 1.2. Figure 1.1 shows a system in which the main gas stream and the powder stream are both introduced into the mixing chamber of the converging–diverging nozzle. This configuration requires that the powder feeder be capable of high gas pressure and is most often used in stationary cold spray systems, for which the cumbersome powder feeder is acceptable. Figure 1.2 shows a system in which the powder stream is injected into the nozzle at a point downstream of the throat where the gas has expanded to low pressure. Generally atmospheric pressure air, drawn by the lower pressure nozzle injection point, is used for powder transport from the feeder. Since this system does not require a pressurized feeder, it is often used in portable cold spray systems. Figure 1.3 shows a typical stationary installation. The nozzle (pointed downward) is directed by a robot arm. The gas heater is attached to the robot arm and is coupled to the side of the nozzle by a flexible, high-temperature hose. The powder feed line enters the nozzle from above.

The ranges of operations for the two systems depicted in Fig. 1.1 and 1.2 are given in Table 1.1. The principal differences separating the stationary and portable systems are the gas pressure and the gas and powder flow rates, with the portable system utilizing readily available compressed air. Stationary systems utilize higher pressure gases and often have a dedicated high-pressure compressor. A low molecular weight gas, such as helium, is sometimes used as the accelerating gas when particles must be brought to very high velocity. Because of the high sonic velocity of helium, attainable particle velocities are often twice those obtained when nitrogen or air are used. The values in Table 1.1 are representative of typical values currently in use but are not necessarily limiting values for the systems.

A wide variety of metal powders can be successfully deposited on metal or ceramic substrates. Amateau and Eden12 list 45 powders that have been successfully cold sprayed at the Advanced Research Laboratory of Penn State, USA. The manufacture and characteristics of powders that are amenable to cold spray are discussed in Chapter 6.

The deposition thickness produced by a moving nozzle can vary from 0.01 to 1.0 mm, depending on powder feed rate, nozzle sweep speed, and deposition efficiency. Multiple coating layers can result in deposits several centimeters thick. The width of a single sweep is approximately 5 mm, and large surfaces can be coated through multiple, slightly overlapping, parallel sweeps. Figure 1.4 shows a deposit of copper powder on an aluminum rod. Nitrogen was the accelerating gas, and the deposit is several layers thick. Upon closer magnification, Fig. 1.5 shows that individual particles have fused into a dense, uniform coating, which adheres tightly to the aluminum surface. The copper is seen to be mixed with the aluminum at the interface.

When a ceramic is used as a deposition substrate, results such as those shown in Fig. 1.6 can be obtained. In this case, nitrogen driving gas was used to deposit copper powder on a silicon carbide substrate. Single lines of copper are deposited as well as a uniform area coating. Ceramics such as alumina, silicon carbide, and aluminum nitride have been successfully coated.

In comparison with other thermal spray coatings, deposits produced by cold spray are characterized by being less porous, and having higher hardness and lower oxide concentration. The Young’s moduli of cold sprayed deposits can be greater than 80% of bulk values.13 The reasons for these characteristics are that cold particles are less susceptible to oxidation and...