Since solid state bonding in the cold gas-dynamic spray process occurs at microscopic levels, the bonding action is not readily observable. Under the proper conditions - when a particle makes intimate contact with the substrate at supersonic speed, the particle will bond to the surface. For improved bonding mechanisms to take place during the cold gas-dynamic spray process, the following conditions are recommended:
One condition necessary for achieving a good coating bond is that there should be minimal contamination to interfere with the intimate molecular contact between the coating and substrate. The obvious contaminants on the substrate surface are dust, dirt, oil, oxides, and paint. These undesirable materials should be removed in advance of spraying by conventional mechanical methods or by using the cold gas-dynamic spray equipment to blast the surface with abrasive.
Oxide contamination also exists on the surface of the feedstock powder particles. With proper manufacturing, storage, and handling of the feedstock powder, this oxide coating will be thin enough to not affect the quality or deposition efficiency of the coating. When the feedstock powder particles impact the substrate, they plastically deform at such high speed that the shattered oxides are driven to the periphery of the splat and are then ejected from the bonding site. This process is pretty similar in results with other processes such as contaminant free explosions or ultrasonic welds.
As with many coating processes, surface texture can be beneficial for maximizing coating bonding to the substrate. Texture increases the contact surface area and may provide some mechanical interlocking. On the other hand, texture can be undesirable for some substrates since the roughness can create propagation sites for micro-cracking.
An increased temperature of the substrate will also improve the deposition efficiency by lowering the impact energy required to form the bond. A significant advantage of the cold gas-dynamic spray process is that the substrate temperature rarely needs to exceed comfortable ambient conditions. There is little chance that the chemistry or the structural bond between the substrate and the feedstock powder is affected at moderate temperatures. If it is necessary to take the chill out of the substrate, this is usually accomplished with the heated gas jet before the feedstock powder is introduced to begin coating. At times, it is recommended to warm up the substrate by spraying air only (and no powder) with the SST equipment. Another benefit of warming the workpiece is that heat helps evaporate moisture and condensation from the workpiece.
To enable coating, the particle speed must exceed a critical velocity; otherwise, the particle will simply bounce off the substrate surface. At moderate velocity the particles begin to erode the substrate surface similar to grit blasting.
When the particle velocity exceeds a critical velocity for the particle and substrate combination, the momentarily high interfacial pressures at impact site allows the atomic structure to come into intimate contact.
Subsequent particle collisions at this same high velocity cause these new particles to plastically deform, compact the already attached particles, and bond to the previously formed layer.
In cases where the velocity of the particle is too high, the particle can be pulverized on impact, resulting in either low deposition efficiency or substrate erosion. Since it can be operated at low critical particle velocities, CenterLine's low-pressure cold gas-dynamic spray equipment can be reliably used for effective spraying of metals such as tin and zinc which are otherwise quite problematic when sprayed at high velocities.
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