Inovati, a Santa Barbara-based company, has successfully developed a low-pressure metal deposition technique that has won it an R&D 100 Award for 2002. Known as Kinetic Metalization, it is capable of depositing a variety of metals as dense coatings onto metal surfaces, without needing prior surface preparation. Copper, stainless steel, nickel, chromium, aluminium, cobalt, titanium, niobium and other metals can all be deposited, as well as alloys based on these metals and braze powders. These coatings can also be sprayed on to ceramic substrates.
Origins of the Technology
The original concept was first introduced by Samuel Thurston in 1902 and was rediscovered by Russian researchers in the 1980s. The process was then developed throughout the 1990s by Inovati, principally by Dr Ralph Tapphorn and Howard Gabel. They perfected and patented the process, developing special apparatus that uses an inert gas to spray metallic powders on to substrates, so eliminating deposition-induced oxide formation.
Equipment Required for Depositing Coating by Kinetic Metalization
Kinetic Metalization is performed with a two-phase, converging-diverging deposition nozzle that accelerates and triboelectrically charges metal particles contained in an inert carrier gas, usually, helium, and a dynamically-coupled debris recovery nozzle that captures surface contaminants and accelerant gas for recycling and reuse.
How the Coating Process Works
Once accelerated to high speed and electrically charged, the particles are directed to a substrate, a mandrel, or into a mould. Subsequent high-speed collision of the metal particles causes very large strain (approximately 80% in the direction normal to impact) in the particles. This deformation results in a huge increase in particle surface area (approximately 4000, producing a new surface that is oxide-free. When these active surfaces come into contact, pure metallurgical bonds are formed.
Using the Process to Abrade and Produce Free Standing Shapes
However, the process is not limited to depositing surface coatings – Kinetic Metalization can be used to produce freestanding shapes. This is achieved via the same process, but the thin coating is allowed to build up through repeated passes of the application gun. The shapes are generated by manipulating the passes of the application gun with a computer-controlled positioning system. First, a description of the part is produced as a CAD file and then the file converted into a stereolithographic file (G Code). This is then used to control the positioning system. The same methodology can also turn the process into a subtractive tool - by spraying a purely abrasive powder, such as SiC, it will act as an efficient abrasive/subtractive spray method.
Advantages of Low Processing Temperatures
According to James Intrater, Principal Engineer at Inovati, the feedstock material for Kinetic Metalization is powdered metal, and since it is deposited at well below its melting point, the coatings exhibit very fine grain size and additionally avoid heat distortion of the item being coated. This is particularly critical in spraying nano-powders and amorphous metals, where in the former the heat will instigate grain growth, and in the latter will cause crystallisation.
Advantages of the Kinetic Metalization Process
Kinetic Metalization has been used to spray very fine grain size powders and amorphous aluminium. It has also sprayed nickel, chromium, niobium and copper alloys while preserving a very fine microstructure, something that thermal spraying, such as HVOF, does not allow. Furthermore, compared to other deposition methods such as electroplating, because Kinetic Metalization is a direct spray jet, masking of the workpiece is not required.
With regards to the type of gas used as a carrier, helium is used as it is inert and can transport the powder at very high speed. Other gases can be mixed with helium to enhance deposition.
The standoff distance from the nozzle to the substrate is typically 1.25cm and the spray nozzles must be made of an abrasion resistant material (e.g. ceramic) and be specifically internally configured to produce the optimum set of spray dynamics. Multiple nozzles can be used in groups or a single nozzle can have a very large diameter, should one wish to coat particularly large areas or numbers of parts.
Kinetic Metalization versus Cold Spraying
Compared to the Cold Spray process developed by Sandia National Laboratories, Inovati has worked out a host of deposition instrumentation and deposition parameters so as to be the first low-temperature spray technology that is fully geared toward production spraying, and capable of very high reliability deposits.
Furthermore, the Sandia National Laboratories patented process is limited to spraying powders below their thermal softening point and requires that all powders be sprayed through a supersonic nozzle. As such, cold spray will exhibit comparatively low deposition efficiencies and inhibits the ability to tailor the properties of coatings and spray-formed objects.