Because solid state bonding in the cold
gas-dynamic spray process occurs on a very small
scale at supersonic speeds, the bonding action
is not readily observable. Whether it can be
seen or not, we know that when conditions are
right and a particle makes intimate contact with
the substrate at supersonic speed, the particle
will bond to the surface. To understand the
bonding mechanisms it helps to look at what
conditions must be present.
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Cleanliness
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of the conditions necessary for achieving a good
coating bond is that there is minimal
contamination to interfere with the intimate
molecular contact between the coating and
substrate. The obvious contamination on the
substrate surface is dust, dirt, oil, oxides,
and paint. These undesirable materials are
generally removed in advance of spraying by
conventional mechanical methods or by using the
cold gas-dynamic spray equipment to blast the
surface with abrasive. |
Oxide
contamination also exists on the surface of the
feedstock powder particles. With proper
manufacturing, storage, and handling of the
feedstock powder, this oxide coating will not be
thick enough to affect the quality or deposition
efficiency of the coating. When the feedstock
powder particles impact the substrate they
plastically deform at such a high speed that the
shattered oxides are driven to the periphery of
the splat and ejected from the bonding site.
This action is much like that which results in
contaminant free explosion and ultrasonic welds.
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Substrate Activation |
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As
with many coating processes, surface texture can
be beneficial for maximizing coating bonding to
the substrate. Texture increases the contact
surface area and may provide some mechanical
interlocking. Texture can be undesirable for
some substrates since the roughness can create
propagation sites for micro-cracking.
The
temperature of the substrate will also improve
the deposition efficiency by lowering the impact
energy necessary to form the bond. A significant
advantage of the cold gas-dynamic spray process
is that the substrate temperature rarely needs
to exceed comfortable ambient conditions. There
is little chance that the chemistry or structure
of the substrate or the feedstock powder will be
affected at even moderate temperatures. If it is
necessary to take the chill out of the
substrate, this is usually accomplished with the
heated gas jet before the feedstock powder is
introduced to begin coating.
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Particle Velocity |
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Another condition necessary to enable coating is
that the particle exceeds a critical velocity.
Below the critical velocity the particle will
simply bounce off the substrate surface. At
moderate velocity the particles begin to erode
the substrate surface similar to grit blasting.
When
the particle velocity exceeds a critical
velocity for the particle and substrate
combination, the momentarily high interfacial
pressures at impact site allows the atomic
structure to come into intimate contact.
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Subsequent particle collisions at this same high
velocity cause these new particles to
plastically deform, compact the already attached
particles, and bond to the previously formed
layer.
In
cases where the velocity of the particle is too
high, the particle can be pulverized on impact.
This action will result in either low deposition
efficiency or substrate erosion. Because
CenterLine’s low-pressure cold gas-dynamic spray
equipment can be operated reliably at low
critical particle velocities it is very
effective for spraying metals such as tin and
zinc that
can be problematic at high velocities.
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