Kinetic Metallization: Background
Kinetic Metallization™ (KM) is a spray process used to deposit a wide range of metals and metal matrix composites onto a substrate. Substrate material may be, ceramic, glass, thermoplastic, or most commonly metal. The deposit may form a coating, which will modify the surface characteristics of the substrate, or, after removal from the substrate, as a freestanding shape. KM feedstock comprises metal powder for metal deposition and metal/ceramic powder mixtures for the deposit of MMCs. Mixtures of dissimilar pure metals or alloys may also be employed. Powder particle size, expressed as average particle size or APS, varies inversely with feedstock density and is in the range of about 20 microns (Al) to 3 microns (WC-Co). Though submicron powders and powders up to 44 microns (-325 mesh) are useful for some applications.
KM is an entirely solid-state process, although a small quantity of heat is applied to the powder, melting never occurs. The heat input is adjusted to a level sufficient to thermally soften the powder and thus enable a large increase in particle ductility. The ductility increase is responsible for the large quantity of deformation that occurs when the particles impact the substrate in the range of 500 to 1,000 m/s.
The illustration above provides a comparison between KM and other deposition processes in terms of state of matter. Note that KM and a related process cold spray are the only solid-state processes. The difference between these processes is that KM uses a specially designed and patented sonic nozzle that produces higher velocity powder particles at a fraction of the cost and complexity of the conventional supersonic nozzle employed by the cold spray variants.
By contrast, the thermal spray variants are a closely related set of spray deposition processes but, deposited from the liquid-state. Thermal spray feedstock is often quire different than KM versions of the same base materials, and generally these differences revolve around the requirement that the particles must be capable of melting. For example, TS uses a larger particle size than KM to resist chemical reaction (e.g., oxidation, decarburization) chiefly with the air that surrounds the molten spray plume. TS processes are often capable of depositing pure ceramic feedstock as well as metals and alloys.
The gas-phase processes depicted on the chart are deposited in a vacuum chamber. The feedstock is in the form of a target that is excited sufficiently to produce a vapor. The vapor is ideally deposited in an atom-by-atom basis. The deposition is not subject to chemical reaction because air is not present in the chamber. But the feedstock looses all of the characteristics of the feedstock. In fact, the morphology and often the lattice structure of the deposition is functionally related that of the substrate than to the feedstock. For example, often the deposition is epitaxial in morphology, which may not be ideal. Deposition rates are slow and limiting thickness is generally a few microns.
Finally, the solution-phase processes are immersion-based techniques. Here the substrate is placed in the solution and either through the imposition of electrical current or galvanic driven current, metallic ions are reduced on the substrate. Like the gas-phase processes, atom-by–atom build up is active with attendant thickness and deposition rate limitations. Major drawbacks include production of environmentally compromising byproducts. The solution-based processes are alone among those on this chart in that they are inherently suited to non-line of site deposition.
Rapid, Massive Deformation -> Huge Increase of Free Surface Area
KM deposition is accomplished through the rapid, massive deformation of particles in the solid-state. Deformation in the direction normal to particle impact is on the order of 80%. Deformation rate is on the order of 10,000 sec-1. This deformation produces an increase in particle surface area of approximately 3 times the original. The newly formed surface is entirely free from oxide and other contaminants and is thus extremely active. Such surface form atom-to-atom (metallurgical) bond with like material on contact. This is the basis for the extremely high quality of KM coatings.