Cold Spray Technique

Abstract

The cold spray (CS) is a solid-state metal particle deposition technique developed by Russian researchers, Alkhimov and Papyrin, et al. (Ide in Light Mater Weld 40:27–34, 2002) during the 1980s. In this technique, metal microparticles are accelerated from subsonic to supersonic speeds using compressed gases, e.g., air, nitrogen, or helium). The particles collide with a substrate in their solid phase without melting, thus forming a film. The current CS technique is classified as the thermal spray (TS) technique, wherein metal or ceramic particles are melted using plasma, laser etc., and collide with a substrate to create a film. However, the CS technique is a completely different process from traditional thermal spraying, as the former does not involve melting the particles, but instead deposits them in a solid state. There are many advantages of the CS technique, some of which are listed below (Japan Welding Society in Welding and joining handbook, Japan Welding Society, 2003; Alkhimov et al in U.S. Patent No. 5, 302, 414; April 12, 1994; The Mechanical Social Systems Foundation, Research report on innovative parts manufacturing using high-speed particle collision 2005; Karthikeyan in International status and USA efforts, ASB Industries Inc., 2004): (1) Ability to apply fine airborne coatings, (2) Ability to inhibit oxidization, heat effect, and thermal stress, (3) Ability to apply thick films (on the order of tens of cm) (Sasaki in Therm Spray Technol 20:32–41, 2000), (4) Ability to apply a compressive residual stress coating (Sasaki in Therm Spray Technol 21:29–38, 2002), (5) Relatively compact nature of the equipment required. Because of these advantages, the CS technique is expected to replace traditional TS technique as a surface reforming technology. As the name implies, CS technique is not a cold processing technique. The term “cold” refers to the processing temperature, from 500 to 1000 ℃ for the working gas, which is lower than that used in traditional thermal spraying (performed at 5000 ℃ (Pattison in Int J Mach Tools Manuf 47:627–634, 2007) in case of plasma spraying). However, temperature of the gas is maintained at a high level to expand the gas and disperse particles at a high speed. During this process, the temperature of the metal particles still remains significantly lower than their melting point. It is possible to deposit the particles without melting or half-melting, and without taking into consideration the effect of significant oxidation on the particles in a solid state. In the TS technique, because the particles are melted and dispersed, residual stress is generated during the cooling process, making it difficult to form thick films. In contrast, it is possible to form thick films with the CS technique, as shown in Fig. 10.1, considering the materials and deposition conditions are conducive. This particular ability is expected to have potential applications not only in producing two-dimensional coatings, but also in three-dimensional molding technology. Figure 10.1 shows a fine copper powder deposited on an aluminum pipe at a depth of 50–60 mm. Later, the flange is processed, the holes are opened, and threads are cut. Though the particles are not melted at the time of deposition, there is a great cohesive strength between them, and machine operations can be performed in a similar way as with standard bulk materials. Figure 10.2 shows a schematic diagram of the CS equipment. The equipment consists of a nozzle to accelerate the particles to collide with the substrate at a high speed, a gas heater for heating the working gas and accelerate the particle by expanding the volume of a gas, and a powder hopper for supplying particles to the nozzle. CS techniques are generally categorized as low-pressure and high-pressure processes based on the pressure and temperature of the working gas. In the low-pressure process, the working gas pressure is maintained at 1 MPa or lower, and the maximum working gas temperature is maintained at a lower temperature of 500 °C (for a specified temperature range of 500–1000 °C). Therefore, in the low-pressure technique, the particle velocity is slower than the high-pressure process, making it difficult for metal materials (with a high melting point) to deposit on a surface. However, the low-pressure process can be applied to deposit relatively soft materials such as aluminum and copper, and the deposit of certain ceramic and polymer materials are also possible, as discussed below. However, as the low-pressure CS equipment produces a relatively slower particle velocity than the high-pressure equipment, it has significantly less efficiency. However, the equipment is very compact and can use the compressed air as a working gas—its two foremost advantages. Hence, the low-pressure process holds great potential to perform on-site repairs. The “high-pressure” process uses a working gas pressure in the range of 1–5 MPa and a maximum working gas temperature of 1000 °C. Using the high temperature and pressure to increase the particle temperature and velocity, it is possible to deposit metal materials with high melting points, such as stainless steel and nickel-based superalloys, with high efficiency. For soft materials such as pure copper, a deposition efficiency above 95% is reported. However, the equipment required to achieve such a specified high pressure is relatively large, and is not suitable to perform on-site repairs, which is a significant disadvantage for the process. High-pressure and low-pressure CS processes have their pros and cons, and it is advisable to select the process for a given task that is based on the purpose and materials. Here, we present an example of a metal particle deposition mechanism featuring aluminum particles deposited by applying low-pressure CS, as well as examples from analyses conducted by ceramic deposition and ultra-high-molecular-weight polyethylene deposition by applying low-pressure CS.