Thermal Spray at Mach 6: A New Level
of Advanced Performance Coatings
By Margaret Broz
19
Technology
Thermal spray technology has been essential to the progress
and innovation of many products and processes since its
inception in the early 1900s. Further developments have led
to a wider use of thermal spray coatings beyond aviation and
national defense industries to include applications in agriculture,
automotive, and other sectors. Innovators have continued
to improve on the original idea to meet increasingly complex
demands for effective, rugged, and versatile coatings.
Engineers at Hypersonix LLC recently commercialized
hypersonic plasma particle deposition (HPPD), a process that
further expands on the concept of thermal spray. It was invented
by researchers at the University of Minnesota, and leveraged by
Winona, Minn.–based Hypersonix. Together with collaborators at
Mesotek Corp. in Toufen, Miao-Li, Taiwan, the company has built
a production-level system that utilizes the HPPD process. 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 into a heated
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 — Fig. 1. Additionally, HPPD coatings can extend
product performance in less obvious wear applications such as
in electrical contact switches, turbine rotor fins, food processing
equipment, and high-speed hydraulic pumps. There are also
biomedical applications for these coatings where a dense, wearresistant
coating applied to medical implants could potentially
revolutionize the industry.
HPPD is a novel but complimentary technique to thermal
spray and chemical vapor deposition (CVD) and is of value to
applications where more specialized coatings are required.
This article introduces the HPPD process and highlights its
features and benefits. From the plasma that initiates the process,
through the nanoparticle coatings created, this article explores
each step of the deposition technique.
A Specialized Plasma
Developers of the HPPD process used a Thermach SG-100
plasma torch, which generates the high-heat plasma necessary
for HPPD. Thermach worked with researchers at the University
of Minnesota to develop a proprietary cathode-anode pairing
that creates the precise environment the process needs to make
coatings composed of crystalline nano-sized particles.
To begin the process, the plasma is directed through a
specially designed nozzle made of a high-temperature ceramic
that can withstand the extreme temperatures of the plasma. This
nozzle protrudes into a large vacuum chamber (Fig. 2) held at a
moderate level of vacuum (~2 torr). As the plasma stream travels
down the nozzle and exits into the vacuum chamber, it expands
then collapses, creating a characteristic “bubble” shape. This shape
of the plasma is essential to the HPPD process.
Fig. 1 — Carbide inserts coated by HPPD (left); uncoated (right).
Coatings applied at Mesotek.
Fig. 2 — An argon plasma exits a ceramic nozzle (right, glowing orange)
into a vacuum chamber. The plasma stream is elongated by the vacuum
and forms a characteristic “bubble” shape. The speed of particles inside
this plasma stream reaches 2000 m/s (about Mach 6).
thermalspray.org SPRAYTIME | 2018 First Quarter