Coating properties can be designed and engineered for a variety of complex service conditions
such as wear, corrosion, heat, fatigue, friction, and
adhesion, or any combination of these. They are applied in multiple passes to form a lamellar structure
that contains varying degrees of porosity and oxide
inclusions. Fabricators can use thermal spray to restore dimensions on worn parts or to apply appli-cation-specific coatings to new parts (see Figure 4).
Invented in the early 1900s, flame spraying is the
oldest thermal spray method and is still widely used
for reconditioning used and worn parts. It is a versatile process with a wide range of powder types
available, including most metal alloys, nonmetal alloys such as ceramics and polymers, as well as self-fluxing alloys known as spray and fuse powders.
Spray and fuse powders are flame-sprayed onto
the part and fuse at about 1,000 degrees C. The
dense coating metallurgically bonds to the base
metal. The resulting homogeneous coating has no
porosity or oxides and high bond strengths.
Plasma spraying (nontransfered arc) deposits dens-
er coatings than flame spraying and is used extensive-
ly in aircraft engine repair and overhaul. The process
can deposit most metal alloys, carbides, cermets, and
abradable (rub-tolerant) coatings. It is used in all in-
dustries when ceramic coatings are required.
High-velocity oxygen fuel/air fuel (HVOF/HVAF)
spraying produces the densest coatings of any thermal spray process. Used for wear and corrosion resistance, HVOF/HVAF sprays carbides, including tungsten carbide and chromium carbide agglomerated
with nickel, cobalt, and other metal combinations.
Weld overlay powders (see Figure 5) are typically
used in advanced welding processes such as plasma transfer arc welding (PTAW) and laser welding.
In PTAW, a high-energy plasma arc is focused and
concentrated to increase arc density, energy, and
temperature. The overlay process uses self-fluxing
alloys of nickel, cobalt, copper, and all wrought materials. Self-fluxing alloys can be blended with tungsten carbide to increase wear resistance.
Self-fluxing alloys use silicon and boron to reduce
the melting temperature. By themselves, these al-
loys offer very good wear protection and, because
of their lower melting point, offer significant advan-
tages when blended with tungsten carbide. Other
alloys used for PTAW include standard ASTM grades
such as 300 and 400 stainless, C276, and INCONEL®
625. These are used for machinable deposits and for
Parts coated with a PTAW-applied overlay can be
found in mineral excavation and processing and
ground-engaging components used in the agricultural industry. Other applications include valves
and valve bodies, pump components, drilling tools,
and pipeline transportation systems.
Meanwhile, overlaying with laser welding has
gained more market acceptance because of technology modifications and lower costs. This laser
process represents the latest advancement in precision coatings for heat-treatable steels. It also uses
similar powders as PTAW overlays. Laser weld overlays offer low heat input and low preheat temperatures and can deposit precise coatings as thin as
0.020 in. on very thin base metals.
Opportunity for Growth
A/M will drive an increase in powder production and
usage. P/M remains the dominant sector for metal
powders in manufacturing, demanding far greater
volume than the thermal spray and weld overlay
Nevertheless, surfacing technologies are well-recognized to extend the service life of critical components and to refurbish worn parts at a fraction of
their replacement cost. This can be a very attractive
argument during economic slowdowns.
New alloy developments, coupled with new processing equipment and methods, will continue to
spur growth in powdered metal manufacturing. According to a 2014 report from TechNavio Insights,
the global thermal spray market alone was worth
$7.5 billion in 2014 and is expected to grow by more
than 8 percent in the next four years. For fabricators,
therein lies opportunity.
Dale Harper is coating business manager and Gary Tydings is coating specialist at Eutectic Canada, 428 Aimé-Vincent, Vaudreuil-Dorion, QC J7V 5V5, Canada, 800-
361-9439, www.eutectic.ca. This year, as a result of the
commissioning of the new gas atomizer and in collaboration with industry partners, the company started producing low-temperature, high-deposition-rate gas atomized powders for use with thermal spray, PTAW, and laser
When the melt stream enters the vessel, the liquid metal becomes atomized, forming a spray of solid-state particles.
Thermal spray processes deposit various metal powders for specific applications.
Atomizers are sized by the melt capacity of the furnace.
Production batches range from 250 kg to 100 tons.
Thermal Spray Powder Type Applications
Flame spray Metal and nonmetal alloys Part refurbishment
Spray and fuse Self-fluxing metals Protective coatings
Plasma spray (
non-transferred arc) Oxides, carbides, abradables Protective coatings on aircraft engines and parts
HVOF/HVAF Carbides and metal alloys Protective coatings for wear resistance and corrosion
Weld Overlays (fusible powder) Powder Type Applications
Plasma transfer arc welding Self-fluxing and metal alloys Protective weld overlay
Laser welding Self-fluxing and metal alloys Protective weld overlay
Metal powders are used for weld overlays with advanced processes such as plasma transfer arc and laser welding.