July/August_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 2 0 5 6 iTSSe TSS iTSSe TSS C old gas dynamic spray (CGDS), or simply cold spray (CS), has evolved from a wind tunnel experimental observa- tion to a sound commercial spray process. In CS, solid micron-size particles are accelerated in a supersonic inert gas jet (typically nitrogen or helium) and impact a substrate at velocity up to 1000 m/s. The resulting high strain rates experi- enced by the particles and substrate lead to intense plastic de- formation and bonding, all at solid state. This differentiates CS from thermal spray processes, allowing the spraying of tem- perature-sensitivematerials due to the absence ofmelting and oxide inclusion. Even when gas temperatures up to 1000°C are used, the de Laval nozzles used in CS rapidly convert thermal energy into kinetic energy. Consequently, the particles experi- ence most of their flight in a gas that is well below their melt- ing temperature. BONDING MECHANISMS Particle/substrate bonding is attributed to mechanical anchoring of particles to intricate surface features of the sub- strate (similar to Velcro, Fig. 1a) or metallic bonding attributed to atomic forces between the particle and substrate at inter- faces (Fig. 1b). It is generally agreed that the metallic bonding mecha- nisms in CS are similar to cold welding (CW) bonding mecha- nisms. Figure 2 shows the schematic of metallurgical bonding in the CW process, proposed by Bay [3,4] . The presence of the native oxide layer prevents intimate contact between the two metallic surfaces (Fig. 2a). Plastic deformation is induced at the interface as a result of the applied pressure, resulting in break- up of oxide layers (Fig. 2b). Freshly exposed metal is extruded into the gaps by high localized pressure (Fig. 2c). The resulting cleanmetal surfaces put into close contact enables creation of a metallic bond (Fig. 2d). Similarly in CS, when the particles impact the substrate at a sufficient velocity, known as critical velocity ( v cr ) [5-10] , the oxide layer on both the particle and substrate break up due to intensive localized deformation at their interface. This creates gaps between the two fresh metallic surfaces. Pressure at this interface, created by the high velocity impact, extrudes fresh material through those gaps and results in intimate contact between the fresh metals thereby enabling atomic forces to cause bonding. COLD SPRAY: WARMING UP TO HOTTER PARTICLES Increasing and controlling particle impact temperature is proving to be beneficial for both process performance and coating properties. Bertrand Jodoin, Daniel MacDonald, Aleksandra Nastic, Deliang Guo, and Saeed Rahmati University of Ottawa, Canada PROCESS PARAMETERS The ratio of particle impact velocity ( v pi ) to critical ve- locity, as proposed by Assadi et al. [11] , defines an important dimensionless parameter ( η ) that influences both particle de- formation behavior and coating characteristics: (Eq. 1) Coating qualities and process performance increase with η [11,12] . Use of elevated CS process parameters (gas tempera- Fig. 1 — Particle/substrate interface showing (a) mechanical anchoring [1,2] and (b) metallic bonding (TEM image). (a) (b) FEATURE 6