Chapter 2 Thermal Spray Fundamentals
EM 1110-2-3401
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Chapter 2
Thermal Spray Fundamentals
2-1. Introduction
This chapter introduces the engineer to the fundamental principles of thermal spray, coating types and
characteristics, thermal spraying processes, and thermal spray uses.
2-2. General Description of Thermal Spraying
Thermal spraying is a group of processes wherein a feedstock material is heated and propelled as
individual particles or droplets onto a surface. The thermal spray gun generates the necessary heat by
using combustible gases or an electric arc. As the materials are heated, they are changed to a plastic or
molten state and are confined and accelerated by a compressed gas stream to the substrate. The particles
strike the substrate, flatten, and form thin platelets (splats) that conform and adhere to the irregularities of
the prepared substrate and to each other. As the sprayed particles impinge upon the surface, they cool and
build up, splat by splat, into a laminar structure forming the thermal spray coating. Figure 2-1 illustrates a
typical coating cross section of the laminar structure of oxides and inclusions. The coating that is formed
is not homogenous and typically contains a certain degree of porosity, and, in the case of sprayed metals,
the coating will contain oxides of the metal. Feedstock material may be any substance that can be melted,
including metals, metallic compounds, cements, oxides, glasses, and polymers. Feedstock materials can
be sprayed as powders, wires, or rods. The bond between the substrate and the coating may be
mechanical, chemical, or metallurgical or a combination of these. The properties of the applied coating
are dependent on the feedstock material, the thermal spray process and application parameters, and
posttreatment of the applied coating.
Oxide inclusions
Pores/voids
Coating
Cohesive strength
between particles
Splat
Adhesion to
substrate
Substrate
roughness
Figure 2-1 Typical cross section of a thermal spray coating
2-3. Characteristics of Thermal Spray Coatings
a. Hardness, density, and porosity. Thermal spray coatings are often used because of their high
degree of hardness relative to paint coatings. Their hardness and erosion resistance make them especially
valuable in high-wear applications. The hardness and density of thermal spray coatings are typically
lower than for the feedstock material from which the coatings were formed. In the case of thermal spray
metallic coatings, the hardness and density of the coating depend on the thermal spray material, type of
thermal spray equipment, and the spray parameters. In general, the higher the particle velocity, the harder
and denser the coating. Particle velocities for different thermal spray processes in descending order are
detonation, high-velocity oxygen flame (HVOF), arc plasma, arc wire, and flame spray. Hardness and EM 1110-2-3401
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density may also depend on particle temperature and the type of atomization gas used. The porosity of the
coating depends also on the thermal spray process, application parameters, and thermal spray material.
b. Corrosion resistance. Metallic thermal spray coatings may be either anodic or cathodic to the
underlying metal substrate. Because corrosion occurs at the anode, anodic coatings will corrode in
corrosive environments and the cathode will not. Anticorrosive coating systems are generally designed
such that the coating material is anodic to the substrate metal. Anodic coatings will corrode or sacrifice to
protect the substrate. In some cases, the corrosion resistance of the thermal spray material itself is
important. For very high temperature applications and for chemical exposures, the thermal spray coating
must be very corrosion resistant. For such applications, the coating provides a corrosion resistant barrier
to protect the substrate. For a more complete discussion of corrosion theory please refer to Chapter 2 of
EM 1110-2-3400.
c. Adhesion. Thermal spray coatings may have very high adhesion. Special coatings, used for wear
resistance, that are applied by thermal spray processes with very high particle velocity can have tensile
adhesions of greater than 34,000 kPa (5000 psi) as measured by ASTM C633 Standard Test Method for
Adhesion or Cohesive Strength of Flame-Sprayed Coatings. Most coatings used for infrastructure
applications have adhesion values comparable to paint coatings. Typical field- and shop-applied zinc,
aluminum, and zinc-aluminum alloy coatings will have adhesion ranging from 5440 to 13,600 kPa (800 to
2000 psi) as measured by ASTM D4541 Standard Test Method for Pull-Off Strength of Coatings Using
Portable Adhesion Testers.
2-4. Types of Thermal Spray Coatings
a. Corrosion resistant zinc, aluminum, and zinc-aluminum alloy coatings. Zinc, aluminum, and zinc-
aluminum alloy coatings are important anticorrosive coatings because they are anodic to steel. In other
words, they corrode preferentially to steel, acting as sacrificial coatings preventing the corrosion of the
underlying steel substrate. Zinc is a much more active metal in this respect than aluminum. On the other
hand, aluminum coatings are harder, have better adhesion, and form a protective oxide layer that prevents
self-corrosion. Alloys of the two metals have properties somewhere in between, depending on the ratio of
zinc to aluminum. An 85-15 (percent by weight) alloy of zinc and aluminum is a widely used thermal
spray coating material because it is thought to have the best combination of attributes from both metals.
b. Polymer coatings. Thermal spray polymer or plastic coatings have been developed for
infrastructure applications. Thermal spray polymers are thermoplastic powders applied by flame or
plasma spray. The polymer must have a melt temperature that is conducive to thermal spray. In addition,
the polymer must not polymerize, degrade, or char in the flame. Thermal spray plastics do not contain
volatile organic compounds and thus are compliant for use in areas with air pollution regulations.
Thermal spray polymer coatings have been used to coat steel under very cold atmospheric conditions
when painting was not practical. Research has been conducted on the use of recycled plastics for polymer
flame spray, and these products show some potential. There appears to be a growing interest in polymer
flame spray for infrastructure applications. The Society for Protective Coatings is developing a
specification for polymer flame spray, and several vendors offer equipment and polymer feedstocks.
c.
Other thermal spray coatings. Other thermal spray coating materials are used for special
applications. Special metal alloy coatings are commonly used for hardfacing items such as wear surfaces
of farm equipment, jet engine components, and machine tools. Ferrous metal alloys are often used for
restoration or redimensioning of worn equipment. Special ferrous alloys are sometimes used for high-
temperature corrosion resistance. Inert ceramic coatings have been used on medical prosthetic devices
and implants such as joint replacements. Conductive metal coatings are used for shielding sensitive
electronic equipment against electric and magnetic fields. Ceramic coatings have also been used to EM 1110-2-3401
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produce very low-friction surfaces on near net shape components. These and other applications make
thermal spray coatings a diverse industry.
2-5. Thermal Spray Processes
Thermal spray processes may be categorized as either combustion or electric processes. Combustion
processes include flame spraying, HVOC spraying, and detonation flame spraying. Electric processes
include arc spraying and plasma spraying.
a.

Combustion processes.
(1) Flame spraying. The oldest form of thermal spray, flame spraying, may be used to apply a wide
variety of feedstock materials including metal wires, ceramic rods, and metallic and nonmetallic powders.
In flame spraying, the feedstock material is fed continuously to the tip of the spray gun where it is melted
in a fuel gas flame and propelled to the substrate in a stream of atomizing gas. Common fuel gases are
acetylene, propane, and methyl acetylene-propadiene. Air is typically used as the atomization gas.
Oxyacetylene flames are used extensively for wire flame spraying because of the degree of control and
the high temperatures offered by these gases. By gauging its appearance, t