In
plasma spraying process, the material to be deposited (feedstock) — typically as a
powder, sometimes as a
liquid,
suspension or wire — is introduced into the plasma jet, emanating from a
plasma torch. In the jet, where the temperature is on the order of , the material is melted and propelled towards a substrate. There, the molten droplets flatten, rapidly solidify and form a deposit. Commonly, the deposits remain adherent to the substrate as coatings; free-standing parts can also be produced by removing the substrate. There are a large number of technological parameters that influence the interaction of the particles with the plasma jet and the substrate and therefore the deposit properties. These parameters include feedstock type, plasma gas composition and flow rate, energy input, torch offset distance, substrate cooling, etc.
Deposit properties The deposits consist of a multitude of pancake-like 'splats' called
lamellae, formed by flattening of the liquid droplets. As the feedstock powders typically have sizes from micrometers to above 100 micrometers, the lamellae have thickness in the micrometer range and lateral dimension from several to hundreds of micrometers. Between these lamellae, there are small voids, such as pores, cracks and regions of incomplete bonding. As a result of this unique structure, the deposits can have properties significantly different from bulk materials. These are generally mechanical properties, such as lower
strength and
modulus, higher
strain tolerance, and lower
thermal and
electrical conductivity. Also, due to the
rapid solidification,
metastable phases can be present in the deposits.
Applications This technique is mostly used to produce coatings on structural materials. Such coatings provide protection against high temperatures (for example
thermal barrier coatings for
exhaust heat management),
corrosion,
erosion,
wear; they can also change the appearance, electrical or tribological properties of the surface, replace worn material, etc. When sprayed on substrates of various shapes and removed, free-standing parts in the form of plates, tubes, shells, etc. can be produced. It can also be used for powder processing (spheroidization, homogenization, modification of chemistry, etc.). In this case, the substrate for deposition is absent and the particles solidify during flight or in a controlled environment (e.g., water). This technique with variation may also be used to create porous structures, suitable for bone ingrowth, as a coating for medical implants. A polymer dispersion aerosol can be injected into the plasma discharge in order to create a grafting of this polymer on to a substrate surface. This application is mainly used to modify the surface chemistry of polymers.
Variations Plasma spraying systems can be categorized by several criteria.
Plasma jet generation: •
Direct current (
DC plasma), where the energy is transferred to the plasma jet by a direct current, high-power electric arc •
Induction plasma or RF plasma, where the energy is transferred by induction from a
coil around the plasma jet, through which an
alternating, radio-frequency current passes
Plasma-forming medium: • Gas-stabilized plasma (GSP), where the plasma forms from a gas; typically
argon,
hydrogen,
helium or their mixtures • Water-stabilized plasma (WSP), where plasma forms from
water (through evaporation, dissociation and ionization) or other suitable liquid • Hybrid plasma – with combined gas and liquid stabilization, typically argon and water
Spraying environment: • Atmospheric plasma spraying (APS), performed in ambient
air • Controlled atmosphere plasma spraying (CAPS), usually performed in a closed chamber, either filled with
inert gas or evacuated • Variations of CAPS: high-pressure plasma spraying (HPPS), low-pressure plasma spraying (LPPS), the extreme case of which is
vacuum plasma spraying (VPS, see below) • Underwater plasma spraying Another variation consists of having a liquid feedstock instead of a solid powder for melting, this technique is known as
Solution precursor plasma spray Vacuum plasma spraying Vacuum plasma spraying (VPS) is a technology for etching and
surface modification to create
porous layers with high reproducibility and for cleaning and surface engineering of plastics, rubbers and natural fibers as well as for replacing
CFCs for cleaning metal components. This surface engineering can improve properties such as frictional behavior,
heat resistance, surface
electrical conductivity,
lubricity, cohesive strength of films, or
dielectric constant, or it can make materials
hydrophilic or
hydrophobic. The process typically operates at to avoid thermal damage. It can induce non-thermally activated surface reactions, causing surface changes which cannot occur with molecular chemistries at atmospheric pressure.
Plasma processing is done in a controlled environment inside a sealed chamber at a medium vacuum, around . The
gas or mixture of gases is energized by an electrical field from
DC to
microwave frequencies, typically 1–500 W at 50 V. The treated components are usually electrically isolated. The volatile plasma by-products are evacuated from the chamber by the
vacuum pump, and if necessary can be neutralized in an exhaust
scrubber. In contrast to molecular chemistry, plasmas employ: • Molecular, atomic, metastable and free radical species for
chemical effects. • Positive ions and electrons for
kinetic effects. Plasma also generates
electromagnetic radiation in the form of vacuum UV photons to penetrate bulk polymers to a depth of about 10 μm. This can cause chain scissions and cross-linking. Plasmas affect materials at an atomic level. Techniques like
X-ray photoelectron spectroscopy and
scanning electron microscopy are used for surface analysis to identify the processes required and to judge their effects. As a simple indication of
surface energy, and hence
adhesion or wettability, often a
water droplet contact angle test is used. The lower the contact angle, the higher the surface energy and more hydrophilic the material is.
Changing effects with plasma At higher energies
ionization tends to occur more than
chemical dissociations. In a typical reactive gas, 1 in 100 molecules form
free radicals whereas only 1 in 106 ionizes. The predominant effect here is the forming of free radicals.
Ionic effects can predominate with selection of process parameters and if necessary the use of noble gases. == Wire arc spray ==