Hypersonic Plasma Particle Deposition
Hypersonic Plasma Particle Deposition (HPPD) was invented by researchers at the University of Minnesota and commercialized by Hypersonix. This method uses a plasma to synthesize crystalline nanoparticles that are deposited as a dense coating via a one-step, vacuum-assisted process. These coatings have exceptional adhesion, hardness, and resistance to fracture due to the deposition process, which involves ballistic impaction of the nanoparticles travelling at speeds near 2000 m/s into a temperature controlled substrate.
Due to the mechanical properties of HPPD coatings, they are well suited for applications in which wear and corrosion resistance is critical, such as cutting tools and carbide inserts for machining. There are also biomedical applications for these coatings where a dense, wear-resistant coating applied to medical implants could potentially revolutionize the industry.
A Specialized Plasma
To make these crystalline nanoparticles, Hypersonix departs from more traditional thermal spray processes and uses high-purity reactive gases that are piped into the system and combined with the plasma. The injected precursors enter the process at temperatures over 4000 K, where they completely dissociate into constituent atoms. The composition of the nanoparticles, and therefore the coating, is controlled by the type of gases used.
Nanoparticles at Mach 6
The thermal energy contained in the particles coupled with kinetic energy provided by their speed induces a brief phase transformation as the crystalline particles impact. Their deformation creates mechanical as well as chemical bonds, which are enhanced by concurrent chemical vapor deposition (CVD) of remaining unreacted precursors within the plasma stream. These factors greatly improve adhesion while creating a dense coating that has deposition rates of around 30 microns/min.
An advantage of using reactive gases is that they can be easily and thoroughly mixed to create unique coating compositions that would be difficult in other thermal spray processes. Another advantage to using gases can be purified to a greater extent than solid feedstocks. HPPD has the capability to feed many gases into the plasma at once, making it possible to create coatings that are composed of many different elements (such as SiTiCN). Additionally, its control of gas flow rates allows even simple coatings to be compositionally fine-tuned.
The Next Generation Coating
Several important differences exist between HPPD and other coating methods that make HPPD a superior coating process. These range from the precursors used to make the coatings, to the coating structure and overall flexibility of the HPPD process. HPPD films have outstanding mechanical properties because they are intrinsically nanostructured. They have enhanced hardness due to the hindrance of dislocation motion, and enhanced fracture toughness due to prevention of crack propagation. HPPD films follow the Hall-Petch relationship, namely that the strength of materials increases with decreasing grain size (up to a certain limit).
HPPD is a one-step, continuous flow process that creates films starting from gaseous precursors. The purity obtainable with these precursors greatly reduces contaminants in HPPD films. Thermal spray, on the other hand, uses powders or wire as feedstock that can contain many impurities. Gases provide another benefit for HPPD in that they can be easily and thoroughly mixed to create custom film compositions. Various chemical gases can be added together to create complex stoichiometries, and control of gas flow allows precise tuning of the films. Also, because gas flow can be turned on and off or switched with little effort, making multilayer films is quite easy.
Other nanoparticle-based films have disadvantages because of the costly purification and cleaning processes required stemming from the particle synthesis. Mechanical milling or pyrolysis often creates polydisperse particles that contain many impurities. Solution-based chemical methods require extensive post-growth processing and cleaning, and often require complicated coating formulations to prevent flocculation or aggregation. HPPD particles have none of these problems as it’s a one-step process that produces monodisperse nanoparticles.
Another unique feature of HPPD films is that the nanocrystalline particles that make up the film retain their crystallinity after deposition. This is different from what happens in most thermal spray processes, where the material being coated is melted and loses its crystallinity, depositing as an amorphous coating.
Finally, HPPD is unique in that it’s intrinsically nanostructured, and that nanostructure extends into three dimensions. Consider the case of a laminate material, which has an internal two-dimensional (layered) structure. While material properties are improved by the layered structure, that improvement is not the same in all directions (it is anisotropic). Because the structure of HPPD films is formed by nanoparticles, essentially a 3-D network, its properties do not depend on the direction of force, making it a superior material.